Browsing by Subject "Graphite"
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Item Compression/injection molding of bipolar plates for proton exchange membrane fuel cells(2009-12) Devaraj, Vikram; Beaman, Joseph J.; Koo, JosephFuel cells are electrochemical energy conversion devices that convert chemical energy to electrical energy efficiently. Bipolar plates form an integral part of a fuel cell and their high manufacturing cost and low production rate have hindered the commercialization of fuel cells. Bipolar plates require high electrical conductivity, strength, chemical resistance and thermal conductivity. This thesis presents efforts to manufacture bipolar plates which meet these requirements using compression or injection molding. Compression or injection molding processes allow cost-effective, large-scale manufacturing of bipolar plates. A variety of material systems for the fabrication of bipolar plates are processed, molded and characterized.Item Controlling Thermal Properties of Asphalt Concrete and its Multifunctional Applications(2014-08-10) Shi, XijunControlling infrastructure temperature, especially flexible pavement, has attracted attention in both industrial and academic societies because: 1) material properties of asphalt, and corresponding structural responses and distresses are temperature dependent and 2) pavement surface temperature directly relates to various environmental or safety problems. This study investigates the feasibility of mitigating the temperature-related problems of civil infrastructures (especially asphalt pavement) by controlling thermal properties of the construction materials. To change thermal properties of asphalt concrete, expanded polypropylene (EPP) pellet and graphite were selected as the additives and mixed into asphalt concrete. Experimental tests are classified into two categories: 1) physical and thermal characterizations of raw materials including Scanning Electron Microscope and heat susceptibility tests, and 2) mechanical and thermal properties of the modified asphalt mixtures via indirect tensile test and hot disk test, respectively. The heat susceptibility test results show that use of EPP as an aggregate replacement is a better choice than the use of the melted-EPP as a binder modifier because it has a good heat susceptibility and is hard to melt at the HMA working temperature. The mechanical performances and thermal properties evaluation results show that by replacing the aggregate with EPP to have 18% by volume of total mixture, the indirect tensile strength was reduced by 17%, and the thermal conductivity and volumetric heat capacity decreased by 32% and 27%, respectively. By adding 4.8 vol. % of graphite, the indirect tensile strength decreased by 20%, and an increase of 43% in thermal conductivity was obtained. To simulate the effect of the thermally modified asphalt mixtures on the surface temperature of pavements and bridges, a series of heat transfer analysis were conducted using the finite difference heat transfer model. In addition, a case study of a building using EPP modified cement concrete was carried out to investigate the benefits of EPP modified concrete as a wall insulation. From the simulation results, it is concluded that adding graphite into asphalt mixture mitigates the urban heat island effect during summer by dropping the maximum surface temperatures of both pavement and bridge (3.1?C and 1.9?C, respectively, with 4.8% graphite), and the graphite modified asphalt concrete can reduce the use of deicing agents during winter by increasing the minimum surface temperature by 0.5?C for pavement and 0.2?C for bridge. On the other hand, adding EPP increases maximum surface temperature by 0.8?C for pavement and 1.0?C for bridge during winter, which show the potential for snow and ice removal application. In addition, the simulation shows that the EPP modified concrete can serve as a wall insulator.Item Design and Construction of a Low Temperature Scanning Tunneling Microscope(2010-10-12) Chen, ChiA low temperature scanning tunneling microscope (LTSTM) was built that we could use in an ultra high vacuum (UHV) system. The scanning tunneling microscope (STM) was tested on an existing 3He cryostat and calibrated at room, liquid nitrogen and helium temperatures. We analyzed the operational electronic and vibration noises and made some effective improvements. To demonstrate the capabilities of the STM, we obtained atomically resolved images of the Au (111) and graphite surfaces. In addition, we showed that the stable tunneling junctions can be formed between the Pt/Ir tip and a superconducting thin film PbBi. We observed the atomic corrugation on Au (111) and measured the height of the atomic steps to be approximately2.53?, which agrees with published values. In our images of the graphite surface, we found both the ? atoms triangular structure, as well as the complete ?-? hexagonal unit cell, using the same tip and the same bias voltage of 0.2V. The successful observation of the hidden ? atoms of graphite is encouraging in regards to the possibility of imaging other materials with atomic resolution using our STM. We also demonstrated that stable tunneling junctions can be formed at various temperatures. To demonstrate this, the superconducting current-voltage and differential conductance-voltage characteristics of a PbBi film were measured from 1.1K to 9K From this data, the temperature dependent energy gap of the superconductor was shown to be consistent with the predictions of the Bardeen, Cooper, and Schrieffer (BCS) theory.Item Development of nanofiber protective substrates(Texas Tech University, 2004-08) Subbiah, ThandavamoorthyElectrospinning uses high voltage electric field to produce high surface area fibers in the nanometer range. Polymeric nanofibers were prepared by the electrospinning process and were characterized using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). A study on the relationship between process parameters and fiber characteristics was undertaken. The dependence of fiber morphology on the solvent volatility and collector substrate characteristics was critically analyzed. Results on the self assembling nature of the charged fibers over different collector substrates were obtained and reported in the thesis. Defect free nanofiber webs with high specific surface area and low porosities suitable enough to be used as adsorptive filtration membranes were prepared. Polyurethane nanofibers were used as nano metal oxide catalyst carriers by successfully impregnating the catalyst in a single-step electrospinning process. Aerosol filtration abilities of nanofiber membranes were tested and the results are presented.Item Nanoscale graphene for RF circuits and systems(2013-08) Parrish, Kristen Nguyen; Akinwande, DejiIncreased challenges in CMOS scaling have motivated the development of alternatives to silicon circuit technologies, including graphene transistor development. In this work, we present a circuit simulator model for graphene FETs, developed to both fit measured data and predict new behaviors, motivating future research. The model is implemented in Agilent ADS, a circuit level simulator that is commonly used for non-standard transistor technologies, for use with parameter variation analyses, as well as easy integration with CMOS design kits. We present conclusions drawn from the model, including analyses on the effects of contact resistance and oxide scaling. We have also derived a quantum-capacitance limited model, used to intuit intrinsic behaviors of graphene transistors, as well as outline upper bounds on performance. Additionally, the ideal frequency doubler has been examined and compared with graphene, and performance limits for graphene frequency multipliers are elucidated. Performance as a demodulator is also discussed. We leverage this advancement in modeling research to advance circuit- and system-level research using graphene transistor technology. We first explore the development of a GHz planar carbon antenna for use on an RF frontend. This research is further developed in work towards the first standalone carbon radio on flexible plastics. A front end receiver, comprised of an integrated carbon antenna, transmission lines, and a graphene transistor for demodulation, are all fabricated onto one plastic substrate, to be interfaced with speakers for a full radio demo. This complete system will motivate further research on graphene-on-plastic systems.Item One-dimensional electron systems on graphene edges(2007-12) Hill, Jason Edward, 1978-; MacDonald, Allan H.In this dissertation several aspects on one-dimensional edge states in grapheme are studied. First, a background in the history and development of graphitic forms is presented. Then some novel features found in two-dimensional bulk graphene are presented. Here, some focus is given to the chiral nature of the Dirac equation and the symmetries found in the grahene. Magnetism and interactions in graphene is also briefly discussed. Finally, the graphene nanoribbon with its two typical edges: armchair and zigzag is introduced. Gaps due to finite-size effects are studied. Next, the problem of determining the zigzag ground state is presented. Later, we develop this in an attempt to add the Coulomb interaction to the zigzag flat-band states. These nanoribbons can be stimulated with a tight-binding code on a lattice model in which many different effects can be added, including an A/B sublattice asymmetry, spin-orbit coupling and external fields. The lowest Landau level solutions in the different ribbon orientations is of particular current interest. This is done in the context of understanding new physics and developing novel applications of graphene nanoribbon devices. Adding spin-orbit to a graphene ribbon Hamiltonian leads to current carrying electronic states localized on the sample edges. These states can appear on both zigzag and armchair edges in the semi-finite limit and differ qualitatively in dispersion and spin-polarization from the well known zigzag edge states that occur in models that do not include spin-orbit coupling. We investigate the properties of these states both analytically and numerically using lattice and continuum models with intrinsic and Rashba spin-orbit coupling and spin-independent gap producing terms. A brief discussion of the Berry curvature and topological numbers of graphene with spin-orbit coupling also follows.